1. Field of the Invention The present invention relates to a variable magnification optical system, for example, a variable magnification optical system suitable for a digital camera and a digital appliance provided with an image input capability that take in an image of a subject by an image sensor (especially a compact zoom lens system that offers high performance), and to an image-taking apparatus provided with such a variable magnification optical system.
2. Description of Related Art
Many of well-known zoom lens systems for use in digital still cameras and video cameras are so-called four-unit zoom lens systems each including positive, negative, positive, and positive lens units. This zoom lens system includes: a first lens unit having a positive optical power; a second lens unit having a negative optical power; a third lens unit having a positive lens unit; and a fourth lens unit having a positive optical power. Patent documents 1 to 5 below have proposed the zoom lens systems aimed at better performance and shorter lens full length by additionally arranging, on the image side of the positive, negative, positive, and positive lens units, a fifth lens unit that is kept in a fixed position during zooming:
[Patent document 1] Japanese Patent Application Laid-open No. H04-78809
[Patent document 2] Japanese Patent Application Laid-open No. H05-19167
[Patent document 3] Japanese Patent Application Laid-open No. H05-241072
[Patent document 4] Japanese Patent Application Laid-open No. H05-264902
[Patent document 5] Japanese Patent Application Laid-open No. H08-327903.
However, in any of the zoom lens systems described in patent documents 1 to 4, the third lens unit mainly involved in convergence process has a relatively small optical power, thus making it difficult to downsize the zoom lens system. In contrast, the zoom lens system described in patent document 5 has been downsized by providing the third lens unit with a relatively large optical power. Spherical aberration and coma that increase as a result of increasing the optical power of the third lens unit are corrected by providing the positive lens element of the third lens unit with an aspherical surface. Since the positive lens element of the third lens unit needs to be provided with a large optical power, the degree of freedom in correcting aberrations by use of the aspherical surface is limited, which results in hindering performance improvement.